1. Field of the Invention
The present invention relates generally to the field of joining the tip of a wire to another surface and, more particularly, to forming a ball at the tip of such a wire preparatory to such a joinder.
2. Description of the Prior Art
In the fabrication of electronic circuits incorporating integrated circuit chips, many secure electrical connections must be made to individual conducting pads on the integrated circuit so as to provide the means to interconnect its myriad circuits with other components. Ball bonding is a technique which has gained widespread acceptance in the electronics industry for effecting such interconnections. U.S. Pat. No. 3,767,101, issued to W. D. Genrich, and assigned to the present assignee, describes this technique and a bonder suitable to perform it. As described in that patent, a wire is fed through the tip of a capillary until it protrudes slightly beyond its end. A flame applied to the wire near the capillary tip terminates the wire at that point and causes it to form a small ball. The capillary is then moved against the conducting pad to which the wire is to be bonded until the ball-tipped wire makes contact therewith. By the combined application of pressure, heat and vibration, a bond is created between the conducting pad and the ball at the tip of the wire. The capillary is then withdrawn, allowing wire to be pulled from it to form a lead tailing from the bonded tip of the wire. The wire may then be terminated to form a free lead, or may be bonded to a second point on the circuit in a manner which is not relevant here.
Gold is the metal of choice in such a bonding arrangement because it does not oxidize when the ball is formed on the wire being heated. Considerable research has been directed to discover ways in which non-precious metal wires can be successfully substituted for gold. Some of these efforts are described in "Ball Bonding Of Nonprecious Metal Wires" in the August 1982 issue of Semiconductor International at pages 65 et seq. According to the article, it is impossible to form a ball at the end of an aluminum alloy wire by flaming off in air. Not only is ball formation difficult under those circumstances, but a subsequent bonding operation is made impossible according to the article by the formation of thick oxide layers on the surface of the ball. The article despribes as a possible solution the performance of the flaming off operation by performing the ball formation in an inert gas, such as argon.
Another approach to forming a ball at the tip of a wire preparatory to ball bonding appears in British Pat. Nos. 1,536,872 and 1,600,021. Both of these patents address the goal of forming a ball at the tip of an aluminum wire and propose to do so by. means of a spark discharge. In Pat. No. 1,536,872 the spark discharge is initiated by applying a voltage of about 200 volts across a gap between the wire end and an electrode, and bringing the tip of the wire and the electrode into temporary contact, fusing the wire's tip and forming the spark discharge while feeding a shielding gas to the region of the spark discharge. A similar technique for forming a ball at the tip of an aluminum wire is described in Pat. No. 1,600,021, with the spark discharge, however, being initiated across the gap in a shielding atmosphere. This apparently was thought to be made possible by the use of a voltage between 350 volts and 10,000 volts.
From the above it is apparent that previous approaches to the formation of a ball at the tip of an aluminum or other base metal wire were limited to the use of an inert atmosphere in order to prevent the formation of an oxide layer on the ball which would prevent subsequent bonding, and that the use of such an inert gas atmosphere was deemed necessary both where the ball was formed by flaming off and by spark discharge. To the best of our knowledge this approach has not produced balls of consistent shape and size, sufficiently free of oxides to allow reproducible bonding results, because the turbulence created by the moving bonder mechanism does not allow an inert gas envelope of consistent density to be maintained in the immediate area of the flame-off or spark discharge.
It is a principal object of the present invention to form a substantially oxide-free ball at the tip of a wire by a method which results in consistent ball size, which is equally effective whether the wire be precious metal or base metal, and which does not necessitate the use of an inert gas atmosphere.
A related object of the invention is to provide apparatus capable of developing and applying the necessary potentials and currents to form, in air, a ball of consistent and controllable size at the end of a wire even though the wire is of a base metal.